Subterranean formations are often stimulated to facilitate increased production of hydrocarbons. Fracturing methods use a fracturing fluid at a pressure sufficient to create a fracture or extend existing fractures in the formation. If a proppant is employed, the goal is generally to create a proppant filled zone from the tip of the fracture back to the wellbore. The hydraulically induced fracture is more permeable than the formation and it acts as a pathway or conduit for oil in the formation to flow to the wellbore and then to the surface. These methods of fracturing are well known and while subject to significant variation, most follow a similar general procedure.
The fluids used as fracturing fluids in such formations are typically fluids that have been viscosified to facilitate fracturing and proppant transport. Viscosification of the fluid is typically achieved through the addition of natural or synthetic polymers, which may or may not be cross-linked. The viscosifying polymer may be a solvatable or hydratable polysaccharide, such as guar. Alternatively, a viscoelastic surfactant may be used to viscosify the fracturing fluid. In either case, such fracturing fluids are relatively costly due to the expense of the various components and additives used.
Amounts of the viscosified fluids can leak off into the formation and may reduce the relative permeability in the invaded region after the treatment. Cleanup of these fluids is therefore an important consideration, which may add to the cost of treatment. Even with effective cleanup, there is always the potential that some formation damage will remain. Therefore, breaker systems are commonly used to reduce the viscosity of the fracturing fluid, and allow removal of the fracturing fluid. Guar may be degraded enzymatically or by oxidative chemistry, and commercially available breaker systems are known to those in the art.
In so-called tight shale or sand formations, fracturing with conventional viscosified fracturing fluids may not be practical due to the expense and risk of damage to the already low permeability of the formation.
Tight shale or sand formations are often stimulated using slickwater fracturing where water is combined with a friction reducing agent, typically a polyacrylamide polymer, and is introduced into the formation at a high rate to facilitate fracturing the formation. Tight shale or sand formations are naturally fractured to some degree, and slickwater fracturing is believed to join natural fractures together to form extended, branched fracture networks. In other formations, slickwater fracturing fluids may produce longer, although more narrow fractures, and also use lighter weight and significantly lower amounts of proppant than conventional viscosified fracturing fluids. Accordingly, slickwater fracturing fluids are particularly useful in low-permeability, gas-bearing formations, such as tight-gas shale and sand formations. The slickwater fracturing fluids may be brine or fresh water, depending upon the properties of the formation being treated, and may also require less cleanup than conventional viscosified fracturing fluids.
While slickwater fracturing fluids may require less cleanup than conventional viscosified fluids, there is still the possibility of fracture or formation damage from the friction-reducing polymer, which typically is a high molecular weight polyacrylamide-based polymer, such as a polyacrylamide/2-acrylamido-2-methylpropanesulfonic acid (AMPS) co-polymer. Synthetic polymers such as polyacrylamides tend to be more difficult to break than natural polymers such as guar, due to differences in the structure of the polymer backbone.
It is known that persulfates or peroxygen compounds can be used to degrade or break synthetic polymers. Persulfates thermally decompose at elevated temperatures (≥35° C.), resulting in highly reactive sulfate radicals which initiate the oxidative degradation of the polymer backbone. It is also known to use metal activators to enhance the oxidative degradation of polymers.
When delay or control of polymer break is required, the most common method is to add the breaker compound as a solid which will dissolve slowly, or to use encapsulation methods. However, fracturing operations tend to be set up for the addition of liquid additives and the metering of solid additives can be inaccurate and result in inconsistent chemical loadings which can cause job failures.
There remains a need in the art for a method of breaking both natural and synthetic polymers in a time controlled manner across a range of temperatures.